The present invention relates to the inspection of header tube hole surfaces and, more particularly, to a new and improved scanning apparatus and method for inspecting surfaces of header tube holes.
Headers are used extensively in steam boilers as a means for joining fluid circuits and for distributing fluid to fluid circuits. Such headers typically comprise a large-diameter, heavy-walled cylindrical shell, circular in cross section, having multiple straight or bent tube stubs which extend partially into holes formed through the wall of the header. The tube stubs are fixed to the header via rolled tube joints or by welded tube connections. The end on the portion of each tube stub extending outside of the header is designed to be connected to a component of a fluid circuit, usually in the form of a tube which is welded to the tube stub.
Cracks that initiate from the header surface surrounding the tube holes in steam headers of boilers in fossil fuel burning electrical power plants can lead to failures which can cause costly unscheduled plant outages. If cracks are detected in early stages, however, plans can be made to repair or replace the header during scheduled maintenance and repair outages. Detection and characterization of cracks near the surface surrounding holes in structural components, by nondestructive testing, is often a key factor in assessing the condition and remaining useful life of a component. Crack characterization, i.e., measurement or estimates of depth, length and location of cracks that initiate from the surface of holes in headers is used with additional information to predict remaining life and to plan repair or replacement of the headers.
Cracks have been detected with boroscopes that use fiber optics technology. Access to the hole is obtained by cutting and removing a section of the tube or tube stub that leads into the header. The boroscope is manually inserted into the hole and rotated to inspect the entire circumference. However, proper focusing of the boroscope is difficult to achieve due to instrument sensitivity to changes in probe-to-surface distance. Therefore, quality and reliability of data is less than adequate without a mechanism which permits a controlled scan that maintains a relatively constant position and alignment of the probe. The same, as well as other difficulties, arise with regard to the use of other sensors including ultrasonic transducers and eddy current probes.